1. Field of the Invention
The present invention relates to a recording medium, and preferably to a magnetic recording medium that is used in an information recording device, especially a hard disk device, mounted in information processing apparatuses such as computers or consumer appliances.
2. Background of the Related Art
The amount of information handled by computers and other types of information processing apparatuses has recently been increasing but the information processing apparatuses continue to become smaller and smaller. In view of such circumstances, due to an attempt to increase the recording capacities of the information recording apparatuses, the recording capacities that are required by the recording media of the information recording apparatuses have been increasing.
In order to increase the recording volumes of recording media and at the same time improve the recording performances thereof, the magnetic spacing, which is the distance between the read/write element of the magnetic head and the magnetic recording layer of a recording medium, needs to be reduced as much as possible. The magnetic spacing is determined based on the thickness of the protective layer of the magnetic head, the flight height of the magnetic head, and the thicknesses of the protective layer and lubricating layer of the recording medium. Therefore, reducing the thickness of the protective layer of the recording medium is considered one of the development challenges of the recording medium. Amorphous carbon called DLC (Diamond Like Carbon) is generally employed in the protective layer of the recording medium.
While practicing reduction of the magnetic spacing as described above, on the other hand, a heat-assisted magnetic recording system has been proposed as new technology for achieving a high recording density of a magnetic recording layer of a recording medium and development activity is underway to be put to practical use. In the heat-assisted magnetic recording system, a magnetic material of high coercivity that can withstand thermal fluctuations is applied to the magnetic recording layer. In order to write information, a laser beam is radiated from the magnetic head to record information, with temporarily lowered coercivity of the magnetic recording layer.
FIG. 1 is a schematic cross-sectional diagram of a recording medium formed using a conventional heat-assisted magnetic recording system. In FIG. 1, a recording medium 1 is configured with a substrate 2, a magnetic recording layer 3, a protective layer 4, and a lubricating layer 5. The substrate 2 is formed from a glass material or an aluminum material with a plated surface and functions as a base material. The magnetic recording layer 3 is a layer in which information is read/written by a magnetic head (not shown). When applying the heat-assisted magnetic recording system, the magnetic recording layer 3 is formed by, for example, sputtering a metallic material such as FePt. The protective layer 4 is a material that is formed on the magnetic recording layer 3 to protect the magnetic recording layer 3 from damage, such as corrosion, abrasion, and impact. The protective layer 4 is generally formed from DLC by means of a sputtering method, plasma CVD (Chemical Vapor Deposition), or the like. The lubricating layer 5 is a layer for covering the surface of the protective layer 4. The lubricating layer 5 is generally formed into a thickness of approximately 1 nm by means of a dipping method or the like using PFPE (perfluoropolyether).
The recording density of the conventional recording medium 1 is approximately 500 Gbits/in2, and the thickness of the protective layer 4 is 2 to 3 nm. The thickness of the protective layer 4 needs to be set at 2 nm or less in order to increase the recording density of the recording medium 1 to 750 Gbits/in2 or more. The thickness of the protective layer 4 needs to be set at 1 nm in order to increase the recording density of the recording medium 1 to 2000 Gbits/in2.
In the heat-assisted magnetic recording system, the temperature of the magnetic recording layer 3 is increased to, for example, 300° C. to 400° C. by laser radiation. The protective layer 4 and the lubricating layer 5 are exposed to the heat as well, which brings about concerns involving decomposition or dissipation of particularly the lubricating layer 5. Replacing the protective layer 4 and the lubricating layer 5 with materials of high thermal resistance would be one of the ideas. Unfortunately, it is extremely difficult to develop the materials that bring together the inherent characteristics of the protective layer 4 and the lubricating layer 5, such as lubricity, corrosion protective properties, and impact protective properties. For this reason, an idea proposed is to introduce an insulating layer between the magnetic recording layer 3 and the protective layer 4 to alleviate the impact of heat without significantly changing the materials of the protective layer 4 and the lubricating layer 5.
Japanese Patent Application Publication No. 2010-153012 (Patent Document 1) describes that the reliability of the carbon protective layer and of the lubricating layer can be ensured by interposing an insulating layer made of a material of low thermal conductivity, such as SiO2, TiO2, or ZrO2, between the magnetic recording layer and the carbon protective layer, to relax the flow of heat generated from the magnetic recording layer. Patent Document 1 has a structure in which a new insulating layer is provided in addition to the carbon protective layer. If the thickness of the carbon protective layer is reduced by the thickness of the newly interposed insulating layer in order to keep the original magnetic spacing that is obtained when only the carbon protective layer is used, the functions of the carbon protective layer, such as corrosion resistance, sliding durability, and head flyability, become deteriorated. This is because the insulting layer is not originally designed to function as a protective layer. Therefore, with the additional thickness of the insulating layer to the thickness of the carbon protective layer to keep the properties of the carbon protective layer, it is difficult to keep the thickness of the entire protective layer at 2.0 nm or less.
Japanese Patent Application Publication No. 2010-3359 (Patent Document 2) describes that the protective layer is configured by a plurality of layers and that the insulation effect can be obtained due to the interface thermal resistance of the protective layer. However, normally a low-density initial growth layer with many structural defects exists in the vicinity of the interface between the layers. The initial growth layer, therefore, has a greater portion of the protective layer configured by a plurality of layers, which inevitably results in an increase of the film thickness in order to obtain the same levels of corrosion resistance, sliding durability, and head flyability.
Japanese Patent Application Publication No. H9-138943 (Patent Document 3) describes that the magnetic spacing can be reduced by reducing the thickness of the protective layer without taking thermal insulation into consideration. Patent Document 3 adopts a two-layer structure of Si/a-C, wherein the Si film has the effect of reducing the initial growth layers in the carbon film (paragraphs 0014 to 0016). Patent Document 3, however, does not mention how to ensure thermal insulation applicable to the heat-assisted magnetic recording system, and therefore does not take thermal resistance into consideration. Thus, an insulating layer needs to be newly employed to the structure, inevitably increasing the magnetic spacing accordingly.
The present invention was contrived in view of the problems described above, and one of a plurality of objects of the present invention is to reduce the thickness of the protective layer thereof to 2.0 nm or less while keeping thermal insulation.